Measurement chambers are used for carrying out measurements on liquids using infrared light, to obtain information on the quantity of liquid and its composition. These chambers are based on a transmitter which emits light on one side of the chamber, and a receiver which receives the light after passage through the liquid, on the other side of the chamber.
An estimation of the quantity of liquid in the chamber is obtained by measurement of the attenuation of light in passage through the liquid filling the chamber. An approximation of the composition of the liquid is achieved by analysis of the attenuation in the spectral domain. Since the different components of the liquid have attenuations dependent on the wavelength, it is possible to find the percentage of each component as a portion of the liquid.
Measurement chambers can be classified into two types:
1) Static--the measurement is achieved when the liquid is static in the measurement chamber. These chambers, known as spectroscopy chambers or spectrophotometer cuvets (available from Sigma Chemicals Corp. USA), are used in chemistry laboratories for spectral analysis of the material components;
2) Dynamic--here the measurement of liquid is achieved during movement. Such a chamber is used in milk flow meters, where an estimation of the quantity of milk in the pipe is achieved by measurement of the attenuation of the infrared light, as described in U.S. Pat. No. 5,116,119 to Brayer.
There are two types of measurement chambers in use today. The first type are transparent measurement chambers, in which the entire chamber is made of material transparent to infrared light. The major problem of this method of measurement is that even when the chamber is filled with fluid, light can pass through the side walls of the chamber (bypass light). To the extent that the transparency of the material to infrared light is low, then the situation is that almost all of the light received in the measurement area is bypass light that does not pass through the liquid at all. This phenomenon causes problems in the dynamic measurement range. For example, if 99.9% of the light is bypass light and one is interested in measuring the reduction with an accuracy of 0.1%, then the general required accuracy is 0.0001%. This level of measurement accuracy is very costly to obtain.
If the measurement is static, the situation can be improved by building a chamber having a small cross-sectional area such that even if the transparency of the liquid is low, it is possible to pass light through a thin layer of the liquid. This is not possible in dynamic measurements where there is a need to maintain continuous flow and high supply rate, since a small cross-sectional area would obstruct the continuous flow.
The second type of measurement chamber is a compound chamber which uses two materials, one transparent to illumination and the second opaque. The chamber is made of opaque material having implanted therein windows of transparent material. This construction insures passage of light via the liquid without the bypass phenomena. Disadvantages of the method include the high degree of manufacturing complexity, the need to use two materials, and the problem of sealing at the connection areas.
Therefore, it would be desirable to provide a simple measurement chamber which does not have bypass light problems.